Synthesis gas (hereinafter referred to as syngas) is a mixture of hydrogen (H2) and carbon monoxide (CO). Syngas can be produced, in principle, from virtually any material containing carbon. Carbonaceous materials commonly include fossil resources such as natural gas, petroleum, coal, and lignite; and renewable resources such as lignocellulosic biomass and various carbon-rich waste materials. It is preferable to utilize a renewable resource to produce syngas because of the rising economic, environmental, and social costs associated with fossil resources.
There exist a variety of conversion technologies to turn these feedstocks into syngas. Conversion approaches can utilize a combination of one or more steps comprising gasification, pyrolysis, steam reforming, and/or partial oxidation of a carbon-containing feedstock.
Syngas is a platform intermediate in the chemical and biorefining industries and has a vast number of uses. Syngas can be converted into alkanes, olefins, oxygenates, and alcohols. These chemicals can be blended into, or used directly as, diesel fuel, gasoline, and other liquid fuels. Syngas can also be directly combusted to produce heat and power.
Since the 1920s it has been known that mixtures of methanol and other alcohols can be obtained by reacting syngas over certain catalysts (Forzatti et al., Cat. Rev.-Sci. and Eng. 33(1-2), 109-168, 1991). Fischer and Tropsch observed around the same time that hydrocarbon-synthesis catalysts produced linear alcohols as byproducts (Fischer and Tropsch, Brennst.-Chem. 7:97, 1926).
Currently, catalytic reactors used for selective formation of alcohols from syngas do not completely consume the reactants CO and H2 in a single reactor pass. Further, these reactors can produce CO2, which dilutes the reactant stream, slows the formation of alcohols, and participates in the reverse water-gas shift reaction, thereby producing water. It is desired that unreacted CO and H2 be recycled to the reactor, substantially without CO2, to prevent buildup of CO2 and subsequent lowering of alcohol yields.
What are needed, therefore, are methods, and apparatus for carrying out the methods, for removal of CO2 from a reaction mixture containing reactants (CO and H2) and products (paraffins, CO2, H2O, alcohols, carbonyl compounds, and olefins), followed by recycle of non-condensable gases from reactor effluent. These methods and apparatus would effectively combine separation and chemical reactions.